A Matter of Trust: How the Brain Processes Reputation

Images of the brain showing activation levels in the caudate, the red & yellow areas highlighted in the center.

We make a lot of decisions that hinge on how much we trust another person. Can you trust your neighbor to bring back the ladder he borrowed, or do you stop by to ask for it? Should you let your son off the hook when he says he “accidentally” stepped on his little sister, or should you send him to his room with no dessert?

Trust, in these instances, is built upon past dealings with those people, or their reputations. And there’s a specific part of the brain is responsible for processing reputation during decisions. In a study published in PLOS One by Margaret Wardle, PhD, research associate and assistant professor in the Department of Psychiatry and Behavioral Neuroscience, functional MRI images showed that a part of the brain called the caudate signals reputation during trust-based decisions, responding most strongly to partners deemed indifferent or unfair.

Wardle worked with Luan Phan, professor of psychiatry at the University of Illinois at Chicago, who collected the data on a group of adults who underwent fMRI scanning while playing a trust game with different partners. Participants played the role of “truster,” and were asked to decide what they wanted to do with a small amount of money. They could keep it for themselves or invest it with the partner and double the amount. In turn, the partner could decide to split the returns equally or keep it all, leaving the truster with nothing.

Participants were told that there were three kinds of partners: one who reciprocated (i.e. split the money) more than 50 percent of the time, one who reciprocated 50 percent of the time (splitting it half the time, and taking it all the other half), and one who reciprocated less than 50 percent of the time.

Each partner was uniquely identified, so after a number of trials they developed a reputation as fair, indifferent or unfair partners. What Wardle and her colleagues wanted to see was how the brain reacted to these different reputations.

Margaret Wardle, PhD

“What we were really interested in is looking at how the brain is signaling differently during the decision period, when you’re making up your mind to trust the person or not,” she said.

The fMRI images showed that the caudate, a small area located near the center of the brain, activated strongly when the subject was dealing with the unfair or indifferent partners, but not with the fair partner. A part of the brain “lights up” on an MRI scan when it’s active and requires a greater flow of oxygen. The more oxygenated blood that flows into an area, the stronger the signal.

The caudate is involved in other kinds of decision-making, and is believed to integrate the motor part of an action into the decision, such as deciding whether or not pulling a lever will produce a reward (think clicking the mouse to refresh Facebook again and again to see if anyone liked your vacation photos).

Wardle said she thinks that the caudate is more active when dealing with both the indifferent and unfair partners because the brain has to work harder to understand why a person wouldn’t act within normal social norms. We expect people to reciprocate when we decide to trust them, so a partner who takes our money and keeps it all throws us for a loop.

“My hypothesis about that was that if you look at how people actually behave in the trust game, an indifferent partner is actually a bad partner,” she said. “A typical person is pretty reciprocal in this game. And so it might just be that there isn’t much distinction between indifferent and really bad. They’re all not normative in terms of what people expect.”

Understanding how the brain processes matters of trust and reputation is important because many conditions, such as social phobia or autism, are characterized by difficulties in building interpersonal relationships and trust. Wardle said knowing the norm helps us understand better when and where these processes go awry. This could lead to better understanding of how medications impact those processes, or therapies that can stimulate activity in that part of the brain.

“It’s important to be able to understand what goes on in a normal brain when you are deciding to trust someone, or when you are realizing that this is a bad bet,” she said. “Then we can understand better how these processes go awry in some of these illnesses.”